1. Field of the Invention
present invention relates to a sheet feeding apparatus and an image forming apparatus, and more particularly, to control of a lowering speed when a sheet stacking plate on which sheets are stacked is lowered.
2. Description of the Related Art
A conventional image forming apparatus such as a printer, a facsimile machine or a copying machine is provided with a sheet feeding apparatus which separates an uppermost one of a plurality of sheets accommodated in a sheet cassette and feeds the separated sheet. The sheet feeding apparatus has a sheet cassette and a sheet stacking plate provided in the sheet cassette such that the sheet stacking plate can be lifted and lowered, and sheets are stacked and supported on the sheet stacking plate. When a sheet is fed and an upper surface of the sheet is lowered, the sheet stacking plate is lifted so that the height of the uppermost sheet is maintained in such a range that the sheet can be fed.
In the image forming apparatus having such a conventional sheet feeding apparatus, when sheets are added or a sheet jam is handled, the sheet cassette is pulled out from an apparatus body. When the sheet cassette is pulled out, support of a lift mechanism is released and the sheet stacking plate is lowered (drops) by its own weight, but noise (impulsive sound) is generated in some cases due to impact that results when the lift mechanism is lowered. Especially when a large amount of sheets is stacked on the sheet stacking plate, a loud impulsive sound is generated.
Hence, as countermeasures against noise which is generated when the sheet stacking plate is lowered, it is proposed to slow down a lowering speed of the sheet stacking plate using a damper (see Japanese Patent Laid-Open No. 8-127434). By slowing the lowering speed of the sheet stacking plate in this manner, it is possible to moderate the impact received by the sheet stacking plate and to reduce the noise.
FIG. 8 illustrates a configuration of a sheet cassette having a conventional damper mechanism. The sheet cassette 34 has a relatively large sheet stacking capacity and large-size sheets can be accommodated therein. A sheet stacking plate 35 is provided in the sheet cassette 34 such that the sheet stacking plate 35 can turn in the vertical direction around a fulcrum 35a. A lifter plate 55 receives a driving force from an image forming apparatus body (not illustrated) and is rotated, and the sheet stacking plate 35 lifts the sheets, by the lifter plate 55, to a position where uppermost one of stacked sheets can be fed.
A chipped-tooth gear 42 is provided on a side surface of the sheet stacking plate 35, and a damper gear 51 is provided on an inner wall surface of the sheet cassette 34. The chipped-tooth gear 42 includes a gear portion 42a which meshes with the damper gear 51, and a chipped-tooth portion 42b which is located below the gear portion 42a and which cannot mesh with the damper gear 51.
FIG. 8 illustrates a state where a large amount of sheets P is stacked and in this state, the damper gear 51 and the gear portion 42a of the chipped-tooth gear 42 mesh with each other. If the sheet cassette 34 is pulled out from the image forming apparatus body in this state, the engagement between a driving force on the side of the body and the lifter plate 55 is released. At that time, the sheet stacking plate 35 is lowered by its own weight, but since the sheet stacking plate 35 is located at a position where the damper gear 51 meshes with the gear portion 42a of the chipped-tooth gear 42, the sheet stacking plate 35 is damped by the damper gear 51 and is lowered gently.
However, when the amount of stacked sheets P is small, the sheet stacking plate 35 turns upward and with this, the chipped-tooth gear 42 is also lifted and the damper gear 51 comes to a position facing the chipped-tooth portion 42b of the chipped-tooth gear 42. That is, the damper gear 51 and the chipped-tooth gear 42 are not connected to each other. Therefore, if the sheet cassette 34 is pulled out when the number of stacked sheets is small, the lowering speed of the sheet stacking plate 35 immediately after the sheet cassette 34 is pulled out becomes fast. Thereafter, if the gear portion 42a of the chipped-tooth gear 42 and the damper gear 51 mesh with each other during the lowering motion of the sheet stacking plate 35, the lowering speed of the sheet stacking plate 35 becomes slow by the damper gear 51.
The conventional sheet feeding apparatus and image forming apparatus include a sheet presence/absence sensor which detects the presence or absence of sheets on the sheet stacking plate by inserting a sensor arm into an opening formed in the sheet stacking plate. If the sheet presence/absence sensor detects that sheets have run out, a user pulls out the sheet cassette from the image forming apparatus body to supply sheets.
At that time, if the sensor arm is inserted into the opening of the sheet stacking plate, there is an adverse possibility that the sheet stacking plate and the sensor arm interfere with each other when the sheet cassette is pulled out. Therefore, according to the conventional apparatus, when the sheet cassette is pulled out, the lowering speed of the sheet stacking plate is set to be fast immediately after the sheet cassette is pulled out, but the lowering speed of the sheet stacking plate can be fast only when the number of sheets stacked on the sheet stacking plate is small, and when a large number of sheets are stacked, the above-described problem cannot be solved.
Further, when a sheet jam occurs, the sheet cassette 34 is pulled out from the image forming apparatus body, but when the sheet cassette 34 is pulled out, the sensor arm is in contact with the uppermost sheet on the sheet stacking plate. When the sheet jam occurs, not only the sensor arm but also a feeding roller located above the sheet stacking plate are in contact with uppermost one of the sheets.
In such as case, if the sheet cassette is pulled out, and the sensor arm and the feeding roller rub against the uppermost sheet, there is the possibility of the roller surface being scarred or the sheet being damaged by the uppermost curled up stacked sheet. For this reason, when the sheet cassette is pulled out, it is necessary to increase the lowering speed of the sheet stacking plate but according to the conventional configuration, the lowering speed of the sheet stacking plate can be increased only when the number of sheets stacked on the sheet stacking plate is small, and the above-described problem cannot be solved when a large amount of sheets are stacked.
That is, according to the conventional configuration using the chipped-tooth gear, the lowering speed of the sheet stacking portion cannot be changed midstream depending upon a position (amount of sheets) of the sheet stacking portion when the cassette is pulled out. It is not possible to increase the speed immediately after the sheet stacking portion is lowered when the cassette is pulled out irrespective of the amount of sheets. If a plurality of dampers or chipped-tooth gears are provided, the above-described problem can be solved, but this increases costs and the problem can be solved only in stages.
The present invention has been accomplished in view of the circumstances, and provides a sheet feeding apparatus and an image forming apparatus capable of increasing a speed when a sheet stacking portion is lowered until the sheet stacking portion is lowered by a predetermined amount irrespective of an amount of stacked sheets.